the concerns about someone tinkering with ebuilds without dealing with the maintainer, or tweaking core packages, etc are exactly what DVCS systems like git are good for. Let them go ahead and produce their tweaks, it does no harm until it gets merged with the main repository. You need people policing such merges, not trying to police the generation of the variants.
If the ebuild maintainers need to be involved in any case, then they should approve the merges that change any e-builds they maintain.
for the core system, you need a benevalent dictator, or a team of such (depending on rate of change and trust)
The key is that you are changing the "gentoo developer" status from being an all-or-nothing type of thing to a degree of trust type of thing, where the amount of trust may be different in different areas.
For example, in kernel development David M is highly trusted to make changes in the networking area, but if he started sending patchsets in that made changes in filesystem, memory management core, syspend/restrore, etc without calling the changes out as such and working with the maintainers of those sections, he would get blasted for doing so. (not picking on David M, just using him as an example)